S ince it began with US-Israeli airstrikes in late February, the Iran war has grounded tens of thousands of flights and pushed jet fuel to its highest price in years. Europe 's jet fuel inventories have fallen 50 per cent, and Goldman Sachs warned this week that stocks could drop below the International Energy Agency's (IEA) critical 23-day shortage threshold in June – with the UK identified as most at risk. Many airlines are facing a financial crunch as a result, with jet fuel averaging $181 a barrel globally, roughly double its pre-war level. Lufthansa has axed 20,000 flights through October. Spirit Airlines collapsed after a government bailout fell through. American Airlines faces $4bn more in fuel costs this year, and Delta is looking at a $2bn spike in the second quarter alone. The crisis has brought renewed attention to the hunt for other fuel sources, and there is one in particular that the industry has been talking about for years but is yet to deploy at scale.…